DSPIC30F2010 datasheet(33/202 Pages) MICROCHIP

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DS70118J-page 33

dsPIC30F2010

4.2

Modulo Addressing

Modulo addressing is a method of providing an auto-

mated means to support circular data buffers using

hardware. The objective is to remove the need for soft-

ware to perform data address boundary checks when

executing tightly looped code, as is typical in many

DSP algorithms.

Modulo addressing can operate in either data or

program space (since the data pointer mechanism is

essentially the same for both). One circular buffer can

be supported in each of the X (which also provides the

pointers into Program space) and Y data spaces.

Modulo addressing can operate on any W register

pointer. However, it is not advisable to use W14 or W15

for Modulo Addressing, since these two registers are

used as the Stack Frame Pointer and Stack Pointer,

respectively.

In general, any particular circular buffer can only be

configured to operate in one direction, as there are

certain restrictions on the buffer start address (for

incrementing buffers) or end address (for decrementing

buffers) based upon the direction of the buffer.

The only exception to the usage restrictions is for

buffers which have a power-of-2 length. As these

buffers satisfy the start and end address criteria, they

may operate in a Bidirectional mode, (i.e., address

boundary checks will be performed on both the lower

and upper address boundaries).

4.2.1

START AND END ADDRESS

The Modulo Addressing scheme requires that a

starting and an end address be specified and loaded

into the 16-bit modulo buffer address registers:

XMODSRT, XMODEND, YMODSRT and YMODEND

(see Table 3-3).

The length of a circular buffer is not directly specified. It

is determined by the difference between the corre-

sponding start and end addresses. The maximum

possible length of the circular buffer is 32K words

(64 Kbytes).

4.2.2

W ADDRESS REGISTER

SELECTION

The Modulo and Bit-Reversed Addressing Control reg-

ister MODCON<15:0> contains enable flags as well as

a W register field to specify the W address registers.

The XWM and YWM fields select which registers will

operate with Modulo Addressing. If XWM = 15, X

RAGU and X WAGU Modulo Addressing are disabled.

Similarly, if YWM = 15, Y AGU Modulo Addressing is

disabled.

The X Address Space Pointer W register (XWM) to

which Modulo Addressing is to be applied, is stored in

MODCON<3:0> (see Table 3-3). Modulo addressing is

enabled for X data space when XWM is set to any value

other than 15 and the XMODEN bit is set at

MODCON<15>.

The Y Address Space Pointer W register (YWM) to

which Modulo Addressing is to be applied, is stored in

MODCON<7:4>. Modulo addressing is enabled for Y

data space when YWM is set to any value other than 15

and the YMODEN bit is set at MODCON<14>.

Note:

Y space Modulo Addressing EA calcula-

tions assume word-sized data (LSb of

every EA is always clear).

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